To impart corrosion-resistant properties to the surface of substrates subject to corrosion, such as metals, typically, the surface is coated with chromium-containing compounds. However, chromium-containing compounds present concerns regarding adverse health and environmental effects. The use of electroactive organic polymers as corrosion-inhibiting coatings has been explored as a potential replacement for chromium-containing compounds. One such polymer is polyaniline which can be prepared by oxidative polymerization of inexpensive aniline and which has a relatively good solubility for solution-coating applications.
Polyaniline in its completely oxidized form is known as pernigraniline, and in its fully reduced form as leucoemeraldine. When there is an equal fraction of oxidized and reduced units in the polymer, it is referred to as emeraldine. Upon doping the emeraldine base with a protonic acid such as hydrochloric acid, the emeraldine base exhibits moderate electric conductivity of about 10 S/cm.
It has also been shown that the non-conductive base form of polyaniline as a coating on cold-rolled steel (CRS) offers good corrosion protection. Small amine compounds also have long been used as corrosion inhibitors.
Oligomeric aromatic amines with amino-terminated end groups are of interest in the polymer field due to their use as monomers for preparing polyamides, polyimides and epoxy polymers. The synthesis of amino-terminated oligomers via conventional routes, when possible, is often very complicated. n,n'-bis(4'-aminophenyl)-1,4-phenylenediamine, a reduced form of n,n'-bis(4'-aminophenyl)-1,4-quinonenediimine has been synthesized by catalytic hydrogenation of n,n'-bis(4'-nitrophenyl)-1,4-phenylenediamine and was used as a trimer of aniline in elucidating the structure-semiconductivity relationship of aniline oligomers. In addition, another method was developed for preparing this oligomer to use it as a building block for achieving a total unambiguous synthesis of polyaniline by way of Schiff base chemistry. Both of these methods involve multiple synthetic steps and reduction of the nitro groups in the precursors. As such, it is difficult to apply these methods generally for synthesizing polyaniline and its derivatives, or for using in industrial applications.
As described in U.S. Pat. No. 4,920,517 of Yen Wei, herein incorporated by reference, aniline polymerization is undertaken by using a small amount of an initiator additive, such as 1,4-phenylenediamine, 1,4-aminodiphenylamine, n,n'-diphenylhydrazine, benzidine and the like. These additives drastically increased the rate of the oxidative polymerization of the aniline monomers. The growth of polymer chains is achieved via electrophilic aromatic substitution on neutral monomers by the oxidized growing polymer chain ends. The polymerization process is neither a classical step nor a classical chain polymerization, but is more of a combination of these processes. The additives function as a chain initiator and the molecular weight of polyaniline can be modified by varying the amount of initiator.